1. Field of the Invention
The present invention generally relates to downhole tools useful for drilling oil, gas and water wells. More specifically, the present invention relates to a downhole drilling tool used to pass through a smaller hole and drill a larger hole.
2. Description of the Prior Art
Various methods have been devised for passing a drilling assembly through an existing cased borehole and permitting the drilling assembly to drill a new borehole that is of a larger diameter than the inside diameter of the existing upper cased borehole. One such method uses an under-reamer, which is collapsed to pass through the smaller diameter existing, cased borehole and then expanded to ream the new, larger diameter borehole for the installation of larger diameter casing. Another method is the use of a winged reamer disposed above a conventional bit.
Under-reamers usually have hinged arms with attached cutters. The tool typically has pocket recesses formed in the body where the arms are retracted when the tool is in a closed state. Most of the prior art under-reamers utilize swing out cutter arms that are pivoted at an end opposite the cutting end of the reamer and are actuated by mechanical or hydraulic forces acting on the arms to extend or retract them. Some examples of these types of under-reamers are shown in U.S. Pat. Nos. 3,224,507; 3,425,500; and 4,055,226.
An example of a hydraulically expandable, concentric reaming tool is the RHINO reamer of Smith International, Inc. The tool includes three cutter blocks that are equally spaced around the tool circumference and carrying PDC cutting elements. The cutter blocks are extended from a collapsed position by hydraulic actuation. The cutter blocks include a stabilizer gauge pad and a formation cutting structure. A lock-up system restricts fluid from actuating the cutter blocks during shoe track drill out.
Another example of a hydraulically expandable, concentric reaming tool is the REAMASTER reamer of Smith International, Inc. This tool is illustrated in U.S. Pat. No. 4,431,065, which describes it as having a tubular body for connection to a drill string and a cutting arm received within a recess in the tubular body. The cutting arm is moved between a retracted position approximately aligned with the axis of the tubular body and a deployed position extending laterally outwardly of the body by a hydraulic plunger that actuates the cutting arms from a fully retracted to a fully deployed position.
An example of a mechanically actuated expandable drill bit that does not use pivoting cutter arms to ream a borehole is shown in U.S. Pat. No. 3,365,010. Blades with cutters ride in opposed, axially oriented channels angled with respect to the axis of the tool. When the blades impact the bottom of the borehole, shear pins retaining the blades are broken allowing the blades to move up the channels thereby expanding out against the borehole wall for subsequent borehole enlargement. A large pin for each blade retains the expanded blades in a desired position to control the gage of the borehole. When the expandable drill bit is tripped out of the borehole, the blades fall down the angled tracks through frictional and gravitational forces.
The under-reamer shown in U.S. Pat. No. 3,433,313 has a tubular body with a sleeve movably positioned therein and adapted to move responsive to the pressure of drilling fluid. Movement of the sleeve deploys the cutters to their cutting position. The sleeve is moved in the opposite direction with a wireline tool to retract the cutters from their cutting position and also stop the flow of drilling fluid to allow retraction of the cutters.
An expandable under-reamer is disclosed in U.S. Pat. No. 6,378,632 having an under-reamer body forming at least a pair of opposed downwardly and inwardly angled slots. Fluid is circulated through the under-reamer body. At least a pair of cutter assemblies housed within the under-reamer body is adapted to engage in the opposed angled slots formed by the under-reamer body. Each cutter assembly consists of a cutter support body having a track at a first end, a piston at a second end, and cutters formed in between the ends. The piston is slides within a sleeve formed in the under-reamer body and extending parallel with the angled slots formed in the under-reamer body. The sleeve is in fluid communication with a control port formed in the under-reamer body. Fluid under pressure, when admitted to the piston sleeve below the piston, drives the cutter assembly upwardly and outwardly along the angled slots to commence an under-reaming operation. A spring means in the under-reamer body retracts the cutter assemblies when fluid is shut off at the control port. The hydraulically operated under-reamer opens a borehole below a restriction that is larger than the restriction itself. The under-reamer has a cutter system with a pair of cutters that engage the formation by traversing upward and outward along a track that is angled with respect to an axis of the under-reamer body. The cutters are forced to the extended position by a piston built into each cutter support. Pressure acting on the piston comes from the pressure differential between the annulus and the drill string during circulation of the drilling fluid.
A related type of tool available from Halliburton Security DBS is the Near Bit Reamer. The tool is designed to open the borehole to a larger diameter than the pilot bit. Once the tool is below the casing shoe, the reamer blades are hydraulically actuated. The Near Bit Reamer is adapted for use just above the drill bit or above a rotary steerable system. Also available from Halliburton Security DBS is the XL2 Series under-reamer. This tool can be provided as an expandable stabilizer and is run in conjunction with an under-reamer for better stability. The arms are opened hydraulically and closed mechanically by a return spring.
Another tool described as an eccentric adjustable diameter blade stabilizer is shown in U.S. Pat. No. 6,227,312. The eccentric stabilizer is adapted for mounting on a bi-center bit having an eccentric reamer section and a pilot bit. A pair of adjustable stabilizer blades is recessed within openings in a housing. The blades are radially extended by a camming action produced upon axial movement. An extender piston causes the blades to radially extend and a return spring causes the blades to retract.
Bi-center bits have been used as an alternative to under-reamers as a downhole drilling tool. The bi-center bit is a combination reamer and pilot bit. The reamer section is disposed up-hole of the pilot bit. The pilot bit drills a pilot borehole and the eccentric reamer section follows the pilot bit reaming the pilot borehole to the desired diameter for the new borehole. A desirable aspect to the bi-center bit is its ability to pass through a small hole and then drill a hole of a larger diameter. The drill out diameter of a bi-center bit is limited by the pass-through diameter and the maximum tool diameter. The maximum drill out diameter is related to these parameters by the equation Ddrill out=2*Dpass-through−Dmax tool. It would be desirable to have a downhole tool capable of drilling to a diameter significantly larger than the pass-through diameter.